Existing exhaust purification systems for internal combustion engines may perform combustion under an excess of air, which exhaust purification system is provided with an NOx storage and reduction catalyst which is arranged in an engine exhaust passage, which NOx storage and reduction catalyst stores NOx when an air-fuel ratio of inflowing exhaust gas is lean and releases stored NOx and reduces the released NOx to N2 or ammonia when inflowing exhaust gas becomes rich, an oxidation catalyst which is arranged downstream of the NOx storage and reduction catalyst in the engine exhaust passage, which oxidation catalyst oxidizes part of NO in inflowing exhaust gas to NO2, and a selective reduction catalyst which is arranged downstream of the oxidation catalyst in the engine exhaust passage, which selective reduction catalyst holds ammonia in inflowing exhaust gas and selectively reduces NOx in the inflowing exhaust gas by ammonia under an oxidizing atmosphere. In some exhaust purification systems, the air-fuel ratio of the exhaust gas which flows into the NOx storage and reduction catalyst is temporarily switched to a rich air-fuel ratio AFRA which is adapted for generating ammonia from the NOx which is stored in the NOx storage and reduction catalyst, whereby an ammonia which is produced by the NOx storage and reduction catalyst is held by the selective reduction catalyst, after that when the air-fuel ratio of the exhaust gas which flows into the NOx storage and reduction catalyst is returned to lean, the NOx in the exhaust gas which flows into the selective reduction catalyst is reduced or purified by the ammonia (for example, see Japanese Patent Publication No. 2009-540189A).
If a ratio of an amount QNO (mol) of NO which is contained in an exhaust gas to a total of the NO amount QNO and an amount QNO2 (mol) of NO2 which is contained in the exhaust gas is referred to as a “NO ratio” (=QNO/(QNO+QNO2)), in a selective reduction catalyst, if the NO ratio of the exhaust gas which flows into the selective reduction catalyst is 0.5, that is, if the amount of NO and amount of NO2 in the exhaust gas which flows into the selective reduction catalyst are equal to each other, a NOx purification rate of the selective reduction catalyst becomes the highest and as the NO ratio becomes smaller or larger than an optimum ratio which is 0.5, the NOx purification rate becomes lower. In Japanese Patent Publication No. 2009-540189A, the NO ratio of the exhaust gas which flows out from the NOx storage and reduction catalyst is considerably high, but the NO ratio is made to fall by the oxidation catalyst and approaches the optimum ratio of the selective reduction catalyst. Therefore, an arrangement of the oxidation catalyst raises the NOx purification rate of the selective reduction catalyst.